1. Field of the Invention
The present invention relates to a barrel temperature control apparatus for an injection molding machine.
2. Description of the Prior Art
Conventionally, when resin supplied into a barrel of an injection molding machine is mixed and melted for injection molding, it has been aimed to improve the product quality (yield) at the molding start and further to prevent the resin in the barrel from being deteriorated at the molding end, both by controlling the temperature of the barrel.
For instance, in a feedback control system as shown in FIG. 1, a barrel of an injection molding machine is heated by an injection molding machine heater section 201; the barrel temperature detected by a temperature detecting section 202 is fed back to a subtraction point 203; a temperature deviation between the detected temperature and a target temperature is obtained; and this temperature deviation and manipulated variables based upon the temperature deviation and the PiD parameters of a controller 204 are inputted to the injection molding machine heater section 201 so that the barrel temperature at the injection molding machine heater section 201 is feedback controlled at each start and stop of the injection molding.
However, where the feedback control system is formed as with tile case of the conventional art system as described above, if the parameters are not so adjusted by the controller 204 that the disturbance suppression characteristics can be optimized at all the times, whenever the injection molding operation is shifted from the molding stop status to the molding start status or from the molding status to the molding stop status, the following problems arise:
That is, when non-melted resin is supplied into the barrel after the injection molding operation shifts from the molding stop status to the molding start status and the plasticizing operation starts, if the disturbance suppression is not sufficient, since the barrel temperature drops abruptly, a long time is required until the barrel temperature restores the original stable target temperature, with the result that it is impossible to obtain a stable molding status. As a result, the quality of the molded products deteriorates and thereby the molded products must be disuse to that extent; that is, resin and time are both wasted.
Further, when the supply of non-melted resin into the barrel stops after the injection molding operation shifts from the molding status to the molding stop status and the plasticizing operation stops in the barrel, if the disturbance suppression is not sufficient, since the barrel temperature rises abruptly, a long time is required until the barrel temperature restores the original stable target temperature, so that the resin temperature within the barrel rises to a temperature beyond a predetermined temperature range. As a result the resin is deteriorated and dissolved.
In practice, however, it is impossible to adjust the parameters in the controller 204 in such a way that the disturbance suppression characteristics can be always optimized. Therefore, conventionally, it has been impossible to improve the product quality at the molding start and to prevent the resin in the barrel from being deteriorated under satisfactory conditions.